<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
  <title>WebGL - Fundamentals</title>
  <link type="text/css" href="../resources/webgl-tutorials.css" rel="stylesheet" />
</head>

<body>
  <canvas id="c"></canvas>
</body>

</html>

<script src="../resources/webgl-utils.js"></script>
<script id="vertex-shader-2d" type="notjs">

  // an attribute will receive data from a buffer
  attribute vec4 a_position;

  // all shaders have a main function
  void main() {

    // gl_Position is a special variable a vertex shader
    // is responsible for setting
    gl_Position = a_position;
  }
</script>

<script id="fragment-shader-2d" type="notjs">

  // fragment shaders don't have a default precision so we need
  // to pick one. mediump is a good default
  precision mediump float;

  void main() {
    // gl_FragColor is a special variable a fragment shader
    // is responsible for setting
    gl_FragColor = vec4(1, 0, 0.5, 1); // return redish-purple
  }
</script>
<script>
  /* eslint no-console:0 consistent-return:0 */
  "use strict";

  function createShader(gl, type, source) {
    var shader = gl.createShader(type);
    gl.shaderSource(shader, source);
    gl.compileShader(shader);
    var success = gl.getShaderParameter(shader, gl.COMPILE_STATUS);
    if (success) {
      return shader;
    }

    console.log(gl.getShaderInfoLog(shader));
    gl.deleteShader(shader);
  }

  function createProgram(gl, vertexShader, fragmentShader) {
    var program = gl.createProgram();
    gl.attachShader(program, vertexShader);
    gl.attachShader(program, fragmentShader);
    gl.linkProgram(program);
    var success = gl.getProgramParameter(program, gl.LINK_STATUS);
    if (success) {
      return program;
    }

    console.log(gl.getProgramInfoLog(program));
    gl.deleteProgram(program);
  }

  function main() {
    // Get A WebGL context
    var canvas = document.querySelector("#c");
    var gl = canvas.getContext("webgl");
    if (!gl) {
      return;
    }

    // Get the strings for our GLSL shaders
    var vertexShaderSource = document.querySelector("#vertex-shader-2d").text;
    var fragmentShaderSource = document.querySelector("#fragment-shader-2d")
      .text;

    // create GLSL shaders, upload the GLSL source, compile the shaders
    var vertexShader = createShader(gl, gl.VERTEX_SHADER, vertexShaderSource);
    var fragmentShader = createShader(
      gl,
      gl.FRAGMENT_SHADER,
      fragmentShaderSource
    );

    // Link the two shaders into a program
    var program = createProgram(gl, vertexShader, fragmentShader);

    // look up where the vertex data needs to go.
    var positionAttributeLocation = gl.getAttribLocation(program, "a_position");

    // Create a buffer and put three 2d clip space points in it
    var positionBuffer = gl.createBuffer();

    // Bind it to ARRAY_BUFFER (think of it as ARRAY_BUFFER = positionBuffer)
    gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

    var positions = [0, 0, 0, 0.5, 0.7, 0];
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(positions), gl.STATIC_DRAW);

    // code above this line is initialization code.
    // code below this line is rendering code.

    webglUtils.resizeCanvasToDisplaySize(gl.canvas);

    // Tell WebGL how to convert from clip space to pixels
    gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

    // Clear the canvas
    gl.clearColor(0, 0, 0, 0);
    gl.clear(gl.COLOR_BUFFER_BIT);

    // Tell it to use our program (pair of shaders)
    gl.useProgram(program);

    // Turn on the attribute
    gl.enableVertexAttribArray(positionAttributeLocation);

    // Bind the position buffer.
    gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

    // Tell the attribute how to get data out of positionBuffer (ARRAY_BUFFER)
    var size = 2; // 2 components per iteration
    var type = gl.FLOAT; // the data is 32bit floats
    var normalize = false; // don't normalize the data
    var stride = 0; // 0 = move forward size * sizeof(type) each iteration to get the next position
    var offset = 0; // start at the beginning of the buffer
    gl.vertexAttribPointer(
      positionAttributeLocation,
      size,
      type,
      normalize,
      stride,
      offset
    );

    // draw
    var primitiveType = gl.TRIANGLES;
    var offset = 0;
    var count = 3;
    gl.drawArrays(primitiveType, offset, count);
  }

  main();
</script>